Recently, lithium secondary batteries are generally employed as electric sources for driving small electronic devices. They are also employed as electric sources for driving potable electronic transmission apparatuses such as small size video cameras, cellular phones, and note-size personal computers. They are also expected as electric sources for motor cars. The lithium secondary battery essentially comprises a positive electrode, a non-aqueous electrolytic solution, and a negative electrode. A lithium secondary battery utilizing a positive electrode of lithium compound oxide such as LiCoO2 and a negative electrode of carbonaceous material or lithium metal is preferably used. As a non-aqueous solvent of the electrolytic solution for lithium secondary batteries, a carbonate such as ethylene carbonate (EC) or propylene carbonate (PC) is preferably used.
In the lithium secondary battery, the positive electrode releases an excessive lithium and the excessive lithium deposits on the negative electrode to produce dendrite, if the battery is overcharged to exceed the ordinary working voltage. Therefore, both of the positive and negative electrodes become unstable. When both electrodes become unstable, the carbonate in the electrolytic solution decomposes upon contact with the unstable electrodes and an exothermic reaction rapidly occurs. Accordingly, the battery abnormally generates heat and safety of the battery lowers. This phenomenon makes increased troubles in the case that the battery generates an electric current of an increased energy density.
Until now, it has been proposed that an addition of a small amount of an aromatic compound to the electrolytic solution is effective to assure the safety under the overcharge condition.
JP-A-7-302614 describes that an organic compound having a molecular weight of 500 or less and a π-electron orbit which gives a reversible oxidation-reduction potential at a potential of noble side relative to the positive electrode potential under the fully charged condition, which is represented by an anisole derivative, is used as an additive for an electrolytic solution.
JP-A-2000-156243-describes that an organic compound having a π-electron orbit which gives a reversible oxidation-reduction potential at a potential of noble side relative to the positive electrode potential under the fully charged condition, which is represented by an anisole derivative, biphenyl, and 4,4′-dimethylbiphenyl, is used as an additive for an electrolytic solution. It is described that the organic compound such as the above-mentioned anisole derivative or biphenyl derivative generates a redox shuttle in the battery, so that the safety of the battery is assured.
JP-A-9-106835 (corresponding to U.S. Pat. No. 5,879,834) describes a measure for assuring safety of a battery under the overcharge condition by increasing an internal resistance in the battery. The increase of an internal resistance can be accomplished using a monomer such as biphenyl, 3-R-thiophene, 3-chlorothiophene or furan in an amount of approx. 1 to 4% to polymerize the compound such as biphenyl at a voltage exceeding the maximum working voltage of the battery.
JP-A-9-171840 (corresponding to U.S. Pat. Nos. 5,776,627 and 6,033,797) also describes a measure for assuring safety of a battery under the overcharge condition, by working an internal current breaker in the battery. The internal current breaker can be worked using a monomer such as biphenyl, 3-R-thiophene, 3-chlorothiophene or furan in an amount of approx. 1 to 4% to polymerize a compound such as biphenyl and produce a gas at a voltage exceeding the maximum working voltage of the battery.
JP-A-10-321258 also describes a measure for assuring safety of a battery under the overcharge condition, by producing an electroconductive polymer in the battery. The production of an electroconductive polymer can be accomplished by using a monomer such as biphenyl, 3-R-thiophene, 3-chlorothiophene or furan in an amount of approx. 1 to 4% to polymerize the compound at a voltage exceeding the maximum working voltage of the battery.
JP-A-10-275632 describes that a nonionic aromatic compound having an alkyl group is incorporated into an organic electrolytic solution of a secondary battery which comprises a linear ester as a main solvent. As the nonionic aromatic compound having an alkyl group, there are mentioned a trimellitic ester, tri-2-ethylhexyl trimellitate, dimethyl phthalate, dibutyl phthalate, butylbenzene (normal, tertiary, or iso), cyclohexylbenzene and toluene.
JP-A-11-162512 (corresponding to U.S. Pat. No. 6,074,777) describes that the addition of biphenyl or the like is apt to lower the battery performances such as cycle property when the battery is subjected to repeated cyclic procedure in which the battery is charged to a voltage exceeding the maximum voltage of 4.1 V, or the battery is charged and discharged at a high temperature such as 40° C. or higher for a long period of time, and that these problems are more apparently noted when the additive is added in an increased amount. This publication further describes that an electrolytic solution containing 2,2-diphenylpropane or its analogous compound is favorably employed for assuring the safety of a battery under the overcharge condition because 2,2-diphenylpropane or its analogous compound polymerizes to generate a gas, resulting in working of an internal current breaker, or to give an electroconductive polymer, resulting in generation of internal short-circuit.
The anisole derivatives and biphenyl derivatives described in JP-A-7-302614 and JP-A-2000-156243 favorably work under the condition of overcharge by utilizing redox shuttle, but give adverse effects to the cycle property and storage endurance. In more detail, the anisole derivatives and biphenyl derivatives gradually decompose when the battery is subjected to the repeated charge-discharge procedure, if they are locally subjected to an elevated voltage in the case that the battery is used at a temperature of 40° C. or higher, or that the battery is used at an ordinary working voltage. Therefore, the contents of the anisole derivative and biphenyl derivatives decrease by decomposition in the course of actual uses of the battery, so that the desired safety cannot be assured when the charge-discharge procedure is carried out after 300 cycle charge-discharge procedure is repeated.
Likewise, biphenyl, 3-R-thiophene, 3-chlorothiophene, and furan which are described in JP-A-9-106835, JP-A-9-171840, and JP-A-10-321258 favorably functions under the overcharge condition. However, as indicated in the aforementioned JP-A-11-162512, they give adverse effects to the cycle property and storage endurance. These problems are more prominently noted when the amount of biphenyl increases. In more detail, since biphenyl or the like decomposes by oxidation at a potential of 4.5 V or lower, the content of biphenyl or the like gradually decreases when it is locally subjected to somewhat high voltage in the course of working at 40° C. or higher or at an ordinary working voltage, resulting in decrease of the cycle life. Further, since the content of biphenyl or the like decreases due to its decomposition, the desired safety is sometimes not assured when the charge-discharge procedure is carried out after the 300 cycle charge-discharge procedure is repeated.
In addition, a battery containing 2,2-diphenylpropane and its analogous compound (which is described in JP-A-11-162512) shows only unsatisfactory safety under the overcharge condition, but the attained safety is higher than a battery having no such additive. On the other hand, it is known that the battery containing 2,2-diphenylpropane and its analogous compound shows a cycle property better than a battery containing biphenyl, but worse than a battery containing no additive. Thus, if a battery showing a cycle property better than that attained by the battery containing biphenyl is required, safety should be sacrificed.
It is an object of the present invention to provide a lithium secondary battery showing high safety under the overcharge condition, and excellent battery performances in cycle property, electric capacity and storage endurance, and further provide a non-aqueous electrolytic solution favorably employable for preparing the lithium secondary battery.